This invention relates to highly heat-resistant polyamide compositions and more particularly to compositions comprising polyamides and phenylene ether resins and to an improved method for the preparation of polyamide--phenylene ether resin compositions.
Polyamides, commonly termed nylon resins, are well known for possessing an outstanding combination of strength, toughness and resistance to solvents. Unmodified nylons are widely used in applications requiring those characteristics where low or moderate loads will be encountered and particularly where exposure to extreme temperatures is not likely. Toughened versions of nylons have found increasing use in automotive applications such as trim parts, and reinforcement of polyamides with glass and glass/mineral combinations has extended their acceptance for such diverse applications as fan blades, valve covers, bicycle wheels and the like. However, for applications that require retention of mechanical properties for long periods of use at elevated temperatures, these compositions are also generally considered to be unsatisfactory. Additionally, even though glass and mineral fillers serve to increase rigidity and reduce the shrinkage tendencies of nylon resins, it is well known that filled compositions exhibit reduced ductility and toughness. Filled compositions also may have poor melt flow characteristics which in turn increases the difficulty of molding such compositions.
One method for improving the properties of thermoplastic polyamide compositions without reducing good flow and processing characteristics has been to blend polyamides with dissimilar resins. However, aliphatic and aromatic polyamides are highly polar materials. They are generally incompatible or at best only poorly compatible with a great many dissimilar resins, and particularly so with much less polar resins such as polyolefins, styrenic resins, phenylene ether resins and the like. Blends of polyamides with such resins often exhibit phase segregation in the melt and poor interphase adhesion, which results in delamination, lower ductility and generally poor mechanical properties in extruded or injection molded parts. A method widely known in the art for overcoming such problems has been to provide more polar radicals or amine-reactive groups in the polymer chains of the less polar resin. Carboxylated polyolefins are known to form improved alloys with polyamides, as shown for example in U.S. Pat. No. 4,362,846, and styrenic resins containing a small amount of copolymerized functional monomer such as acrylic acid, maleic anhydride or an epoxy compound become grafted with polyamide when the two resins are melt-processed together, as is disclosed in U.S. Pat. Nos. 3,668,274 and 4,221,879. Processes for introducing reactive functionality such as carboxyl groups into phenylene ether resins are also well known. These modified resins are said to be useful for preparing polyphenylene ethers having chemically linked polyamide chains, as is disclosed in U.S. Pat. No. 3,259,520. Treating preformed polyphenylene ethers with a combination of a styrenic monomer and maleic anhydride in the presence of a free-radical initiator is shown in U.S. Pat. No. 4,097,556 to provide polyphenylene ether-styrene-maleic anhydride graft copolymers which are said to be useful in blends with polyamides. Processes for directly attaching maleic anhydride to phenylene ether resins in the presence of a peroxide are shown in published Japanese application Nos. 59/66452 and 59/59724. Blends of these maleated phenylene ether resins with polyamides are also disclosed therein.
Processes are thus now available for producing alloys of nylons with a variety of dissimilar resins, resulting materials having improved impact, reduced shrinkage and better oven warpage characteristics. Of particular interest for extended use under very severe high temperature conditions are blends of nylons with very high temperature resins such as polyphenylene ethers. These compositions exhibit good solvent resistance and, depending upon the ratio of the components, may possess useful high temperature resistance and good physical properties.
Although the methods available for preparing alloys of phenylene ether resins and polyamides appear to be successful, further improvements are needed. Chemical modification of phenylene ether resins, either by use of functional comonomers or in a post-reaction, requires additional and costly process steps. The methods presently known for directly modifying phenylene ether resins generally require extended mixing times at melt processing temperatures and/or the use of free-radical compounds, conditions which tend to promote crosslinking and/or deterioration of the resin. Extended mixing at high temperatures also increases energy consumption and adds to production costs. An improved method for preparing such alloys which substantially reduces processing times and minimizes resin cross-linking and degradation is needed.